Extension foundations and alternatives to bricks?

Hi Lads and Lasses,

We will be going down the planning permission route early in the new year to see if we can build on top of half of our existing rear extension so that we can have three double bedrooms.

We have thought through the loft conversion route but after initial discussions with the planning office - they advised that it would more than likely be declined as since we would need a side dormer for the stairs then this would "unbalance the semi's"??!?!?!? - So we now want to go the extenesion avenue. Here are the plans ideas below:

Photo of existing:

Drawing (not to scale!!) of existing:

Drawing of what we want:

Apart from getting the foundations inspected and more than likely underpinned - with a cost being substantial I suspect, can you do a timber frame extension and outside render to keep the weight down without up rating the foundations? Is it advisable?

Any reponse (with in reason ) appreciated

what makes you think the existing found's will be inadequate?

i personally would not want a simple timber frame only with no masonry at all.

i'm sure with the right amount of celotex and adequate sheathing you could construct a decent living 'cell'. noise may be a factor though - keeping it in and keeping it out.

all the timbers would need treating and rendering the outside will not be a simple job, what with all the metal lathing and such.

the found's will still have to carry the roof though.

How old is the existing extension? It may well have suitable foundations.

A timber frame is possible, but a BCO may still want foundations as if for a brick construction.

A timber frame will be equal or better than brick in terms of thermal and acoustic insulation - it just depends on how you specify it. Normally you would use either CLS for the frame either 38 x 140 for rockwool, or 38 x 89 for celotex insulation and some across the inside face. Then line internally with a suitable plasterboard. Externally, you can clad it with timber or render on a propriatory mesh support.

The only practical difference with timber frame or masonry, is what is termed its thermal response. Basically a timber frame will heat up and lose heat quickly, whereas a masonry extension will heat up slower but retain some heat in the structure. But insulation wise,they will perform similarly and one will not cost more to heat than the other.

BTW, what is going on with your proposed extension roof? On plan it does not seem like it will work - the hip which attaches to the main roof hip is at a peculiar angle and does not seem like it can be built like that. Have you just drawn it in for convenience?

^woody^ said:

How old is the existing extension? It may well have suitable foundations.

A timber frame is possible, but a BCO may still want foundations as if for a brick construction.

A timber frame will be equal or better than brick in terms of thermal and acoustic insulation - it just depends on how you specify it. Normally you would use either CLS for the frame either 38 x 140 for rockwool, or 38 x 89 for celotex insulation and some across the inside face. Then line internally with a suitable plasterboard. Externally, you can clad it with timber or render on a propriatory mesh support.

The only practical difference with timber frame or masonry, is what is termed its thermal response. Basically a timber frame will heat up and lose heat quickly, whereas a masonry extension will heat up slower but retain some heat in the structure. But insulation wise,they will perform similarly and one will not cost more to heat than the other.

BTW, what is going on with your proposed extension roof? On plan it does not seem like it will work - the hip which attaches to the main roof hip is at a peculiar angle and does not seem like it can be built like that. Have you just drawn it in for convenience?

Click to expand...

Thanks for the response chaps.

I am not sure whether the existing foundations will be suitable (Extension done in 1987) - do you have different levels of foundations for a single and a double extension?? - I don't know - just an assumption on my behalf. The extension already supports a roof so therefore must be suitable, I have assumed that the additional load on the existing foundations would be minimal with a wooden frame, but much heavier with brick/block- just don't want to have to go down the underpinning route - as the patio will most likely be ripped up in the process!!! A another assumption by myself - again not sure of the process

The plans we inherited from the previous owners say the the foundations where agreed on site with the BCO and no actual measurements, which is very frustrating. My wife has worries about a timber frame and the keeping warm element - this extension would face the brunt of the wind and rain that come our way, encouraging to see the insulation details though.

The roof was a quick idea as no idea how the arrangement would work - Am hoping that the architect or similar will have a much better idea of what roof structure and the finished look should be. The planning Office - which this proposal comes from originally - said that flat roofs where not permitted where we live so hence a concoction was born

Foundations for single and two-storey extensions are the same. A 1987 extension will most likely have suitable foundations for the second storey (1m deep). You may just have to dig an inspection hole for the BCO the check. Or dig one now so that you can progress your plans.

You'll need a magician, not an architect for the roof

A timber frame will have better insulation than a masonry wall of the same thickness, or equal insulation qualities in a thinner wall